While both Andrew & Axel is correct in their solutions, it is important to note that marketing cloud has two different send architectures seen below. The important thing to understand is that guided sends / content builder send operate from lists, groups or sendable data extensions. Triggered sends typically have the email passed in the invocation of the send (though something the email is retrieved from a DE or all subscribers), while Journey Builder can send to the email found in a data extension or in the channel address order
What this means from a solution perspective is that:
- When creating sendable data extensions with email address fields, you need to ensure that take the value from "Preferred Email" and fill that value in the email field in the sendable DEs
- The preferred email should be the top priority in the Channel Address Order, to ensure that journey builder always sends to this one when you are not sending to the email from the Journey Builder Entry DE
- Triggered Sends, through which an email could change, should be allowed to update the All Subscriber List (a setting on the triggered send) (e.g. a confirmation email from changing your profile)
- Journey Builder Sends will automatically update the email in all subscribers (if memory serves, it used to be a setting though)
- Depending on your setup, process around it and frequency of email changes, I would recommend one of the following three solutions:
- 1) Any change in email address, should trigger a confirmation email for the new address (and ideally this is double opt-in to confirm new address), this would overcome any challenge you might have (but depends on masterdata model, as to whether it is viable or not
- 2) Alternatively, you can have an API call towards the all subscriber list and BU, to update the email there
- 3) If neither is possible, create an automation where you create a file (data export), export the file to ftp, move it to import folder and import it to all subscribers (a delta file of all who updated profile or email since last run, should be sufficient)
The key thing to observe here:
If you do not plan out a way to update all subscribers, when you are doing guided sends, this will not update the all subscriber list and you therefore risk sending to the "last" email stored in All Subscribers.
I cannot remember if content builder sends allow for the update of email addresses - there is no specific field that says whether it does it or not, so it would have to be tested.